Hi Borr,
Which app are you talking about, and what are you using it for? I'm assuming Canon Connect, but that might be wrong. The only thing I use that app for is getting GPS coordinates. If that's what you're using it for, your phone needs to see the sky. Same with all GPS apps. If you're using it for something like remote shooting or image transfer, it should work in a cave. As long as you can pair your camera with your phone over bluetooth, if you're in range, it should work even in confined spaces.
Your cell signal doesn't really have anything to do with it. As far as I know, you're not going out to the internet.